The present invention relates to hydraulic systems for driving the radiator cooling fans of vehicle engines, and more particularly, to such systems in which the pump supplying the pressurized fluid to the system is of the variable displacement type.
Although it will become apparent from the subsequent description that the present invention has various uses and applications, it is especially advantageous when used to drive the radiator cooling fan of a vehicle engine, and will be described in connection therewith.
Originally, radiator cooling fans were driven directly, i.e., by some form of mechanical connection between the fan and the engine crankshaft. The resulting proportionality between fan speed and engine speed may be desirable at lower engine speeds (e.g., below 3,000 rpm), but is undesirable at higher speeds where additional air flow through the radiator becomes unnecessary, wastes engine horsepower, and creates excessive noise. More recently, viscous fan drives have been developed which overcome the above-mentioned problems of excessive fan speed at higher engine speeds. Viscous fan drives of the type shown in U.S. Pat. No. 2,948,268, assigned to the assignee of the present invention, have an inherent torque-limiting characteristic such that the fan speed is generally proportional to the engine speed up to a certain speed such as 2,500 rpm, then the fan speed levels off and remains constant as engine speed and torque continue to rise. The resulting graph of fan speed versus engine speed has become known as the "viscous curve", and operation in accordance with the well known "viscous curve" is now generally a requirement of fan drive systems.
The conventional fan drive arrangements can be used only with a standard in-line engine, i.e., one having the crankshaft oriented axially. However, in recent years, there has been increasing use by the auto manufacturers of transverse mounted engines, providing front wheel drive. The attempts by those working in the art to provide a satisfactory fan drive system for use with transverse mounted engines has included many different approaches, including mechanical gear trains, electric motors, and flexible shafts.
Another major approach to the cooling of transverse engines is the use of hydraulic systems, including a hydraulic pump driven by the engine, and a hydraulic motor connected to the fan. Those attempting to design a satisfactory hydraulic fan drive system have tried to reduce the space, weight, and cost of such systems by utilizing at least one of the hydraulic components in at least two different vehicle hydraulic systems. For example, there have been frequent attempts to utilize the power steering pump to provide pressurized fluid to operate a hydraulic fan motor, as well as the power steering gear (see U.S. Pat. No. 2,777,287).
One design approached such systems has been to place the fan motor in series with the power steering gear, but upstream therefrom, such that the flow through the fan motor also passes through the steering gear (see U.S. Pat. No. 3,659,567). A major drawback of such prior art systems has been a constant flow rate through the fan motor over all engine speeds from idle to maximum, such that the fan speed is constant regardless of engine speed. Such systems usually require a relatively high fan motor pressure and pump horsepower at lower engine speeds when the pressure drop across the power steering gear is greatest, thus making it difficult to satisfy the pressure and flow requirements of both the fan motor and steering gear simultaneously.